Optical coupling device and light-receiving circuit of same

ABSTRACT

In a light-receiving circuit  31  of an optical coupling device in which a dummy photodiode D 2  is provided in a vicinity of a photodiode D 1,  and their outputs from current-to-voltage converting amplifiers A 1  and A 2  are compared with each other and subjected to waveform shaping by a hysteresis comparator  33  for improving a common mode rejection ratio, negative feedback circuits of the amplifiers A 1  and A 2  include impedance variable circuits Z 1  and Z 2  in which impedances decrease as a level of an inputted photoelectric current Ipd increases. Therefore, an increase of the current Ipd decreases gains of the amplifiers A 1  and A 2  and thus narrows a band, whereas a decrease of the current Ipd increases the gains and thus widens the band. This minimizes pulse width distortion caused by quantity changes of incoming light, thereby realizing a high-speed transmission rate.

FIELD OF THE INVENTION

[0001] The present invention relates to an optical coupling device,which is realized as a photocoupler, for example, and a light-receivingcircuit of the same, in particular to a high-speed light-receivingintegrated circuit on a secondary side.

BACKGROUND OF THE INVENTION

[0002]FIG. 6 is a cross-sectional view showing an arrangement of atypical photocoupler 1. The photocoupler 1 converts an electric signal,which is inputted from a terminal 2 on a primary side, into an opticalsignal by a light emitting integrated circuit 3 on the primary side,converts the optical signal back into an electric signal by alight-receiving integrated circuit 4 on a secondary side, and outputsthe electric signal from a terminal 5 on the secondary side. Thiselectrically isolates a circuit on the primary side from a circuit onthe secondary side, thereby realizing sending and receiving of a signalwhile the devices are electrically insulated from each other. A lightemitting element, such as a light emitting diode, on the light emittingintegrated circuit 3 and a light-receiving element, such as aphotodiode, on the light-receiving integrated circuit 4 are placed in avicinity to face each other. A gap between the elements is filled withtranslucent epoxy resin 6 having a predetermined dielectric constant.Further, their outside is sealed with epoxy resin 7 having lightblocking effect.

[0003]FIG. 7 is a block diagram showing an electrical arrangement of aphotocoupler 11 of a conventional art. The circuit on the primary sideis composed of a sending driver IC 12 and a light emitting element 13,whereas the circuit on the secondary side is composed of a receiving IC14. In the sending driver IC 12, an amplifier 15 converts a voltagesignal, which is inputted to an input terminal IN, into a currentsignal, and a drive element 16 drives the light emitting element 13 toturn on by using the current signal, where a voltage between terminalsVcc 1 and GND 1 is a power supply voltage. Further, the circuit on theprimary side may be composed of only a light emitting element forconverting the inputted electronic signal into the optical signal.

[0004] In the receiving IC 14, a light-receiving element 17 converts theoptical signal into a current signal, where a voltage between terminalsVcc 2 and GND 2 is a power supply voltage. The signal is then convertedfrom current to voltage (hereinafter referred to as I/V conversion) by acurrent-to-voltage converting amplifier (hereinafter referred to as anI/V converting amplifier) 18, and is subjected to waveform shaping by acomparator 19, and outputted to an output terminal OUT.

[0005] Here, a pulse width distortion characteristic is an importantcharacteristic for characterizing a photocoupler. Recently, FA (FactoryAutomation) devices especially have higher performance, for example,because a semiconductor has higher performance, digital devices are morewidely used. This requires a photocoupler to have a high speed, whichinsulates between units of an AC servo or a programmable controller forthe purpose of reducing noises, and protecting the devices. For example,a photocoupler with a transmission speed of 25 Mbps is required to haveno more than ±6 nsec of the pulse width distortion when the pulse widthis a 40 nsec.

[0006] On the other hand, due to unevenness in a quantity of lightoutputted by the light emitting element 13, manufacturing unevenness(unevenness caused during a manufacturing process) in distance betweenthe sending-side circuit and the receiving-side circuit caused at aprocess of molding with the epoxy resins 6 and 7, and the like, aquantity of incoming light into the light-receiving element 17 isconsiderably changed. Furthermore, there is unevenness in a gain of theI/V converting amplifier 18 caused by manufacturing unevenness of thereceiving-side circuit. For realizing a photocoupler having high-speedperformance, it is necessary to minimize distortion of output pulsewidth, which is caused by quantity changes of incoming light into thelight-receiving element 17.

[0007] Moreover, another important characteristic of characterizing aphotocoupler is a common mode rejection ratio (CMRR). The CMRcharacteristics indicates how difficult it is to faultily operate bydisturbance noise. As shown in FIG. 6, the photocoupler 1 has acondenser structure, in which the epoxy resin 6 having a predetermineddielectric constant fills between the integrated circuits 3 and 4, sothat the integrated circuits 3 and 4 are connected by a parasiticcapacitor thereof. Accordingly, when the input side and the output sideof the photocoupler 1 receive steep noise in which a rising and afalling of the pulse are (dv/dt), a noise current of C·(dv/dt) flowsbetween the input side and the output side, where the parasiticcapacitor is C. Then the noise current causes the faulty operation, whena part of the noise current flows into the light-receiving element onthe light-receiving integrated circuit 4.

[0008] One of methods to prevent the faulty operation is a method inwhich the light-receiving element is covered with a transparentconductive film such as an ITO film, and its potential is grounded to aGND potential on the receiving side. In such an arrangement, the noisecurrent caused by the parasitic capacitor flows into a GND on the outputside via the transparent conductive film, and the light-receivingelement receives only the optical signal of the input side. Thisprevents the faulty operation due to noise, and thus realizing high CMRcharacteristics. However, this causes a problem that the process becomescomplicated because it requires a specialized processing device forforming the conductive film.

[0009] Therefore, another method to prevent the faulty operation causedby the parasitic capacitor is an arrangement to employ a dummyphotodiode, as disclosed in Japanese Patent No. 2531070 (publicationdate: Sep. 4, 1996), for example. FIG. 8 is a block diagram showing alight-receiving circuit 21 of another conventional art using such adummy photodiode. The light-receiving circuit 21 is provided with twophotodiodes d1 and d2 having identical properties in an identical shapeand quantity. Only the photodiode d1 is used for receiving the opticalsignal from the light emitting element, whereas the other photodiode d2is shielded from light to be used as a dummy photodiode. The dummyphotodiode d2, having its light-receiving face covered with a cathodemetal wiring 22, is shielded from light with a cathode potential.

[0010] The photodiode d1 and the photodiode d2 are positioned in a crossmanner having a checker-board like arrangement, as shown in FIG. 9. Inaddition, the photodiodes d1 and d2 have an area of approximately0.1×0.1 mm, which is sufficiently small, whereas frames on which theintegrated circuits 3 and 4 are mounted have a size of, for example, 2×2mm. This makes the noise currents flown into the photodiodes d1 and d2substantially identical.

[0011] Therefore, output currents from the photodiodes d1 and d2 aresubjected to the I/V conversion respectively by the I/V convertingamplifiers a1 and a2, and compared with each other by a hysteresiscomparator 23, which is a differential amplifier, and thus the outputfrom the photodiode d1 is subjected to waveform shaping into a pulsesignal. This eliminates a common mode noise component, thereby realizingthe output of high CMR characteristics.

[0012] However, the I/V converting amplifiers a1 and a2 are amplifiersfor linear amplification subjected to negative feedback by resistors r1and r2 as well as condensers c1 and c2. Therefore, for minimizing thedistortion of the output pulse width caused by the quantity changes ofincoming light, a first stage of the amplifier needs to have asufficiently wide band. However, there is a problem that the CMRcharacteristics are deteriorated, when the band of the amplifier iswide.

[0013] Namely, the photocoupler of high speed and high CMR having thetransmission speed of 25 Mbps has an objective to achieve that CMRtolerance is 10 kV/μsec and Vcm=1000V (here, the wording “CMR tolerance”means a level of CMR up to which the photocoupler can tolerate the noisein the common mode noise signal). In this case, as shown in FIG. 10(a),where a rise time of a noise pulse is 100 nsec, its pulse height valueis 1 kV. As a result, a noise current waveform flown to the photodiodesd1 and d2 by coupling primary and secondary capacitors has a pulsewaveform of 100 nsec, as shown in FIG. 10(b). Therefore, since the noisecurrent waveform includes a high-frequency component of 10 MHz or more,when the band of the amplifier is widened more than a band correspondingto the 25 Mbps, the high-frequency component is easily amplified, andthus easily causing the faulty operation due to noise.

[0014] For this reason, the amplifier band cannot be used for obtainingthe CMR characteristics, so the quantity of incoming light into thephotodiode d1 is required to be constant for obtaining the CMRcharacteristics. This narrows an allowance for the manufacturingunevenness, thereby causing a problem that a photocoupler of high speedand high CMR is difficult to be manufactured with a sufficient yield.

SUMMARY OF THE INVENTION

[0015] The object of the present invention is to provide an opticalcoupling device having a high-speed transmission rate of a pulse,wherein the optical coupling device minimizes pulse width distortioncaused by quantity changes of incoming light directed into alight-receiving element, without being made susceptible to faultyoperation due to noise.

[0016] In order to achieve the object, an optical coupling device of thepresent invention is so adapted that a circuit on a primary side uses alight emitting element to convert an inputted electric signal into anoptical signal, whereas a circuit on a secondary side, in which alight-receiving element is placed in a vicinity of the light emittingelement so as to face the light emitting element, uses thelight-receiving element to accept the optical signal and to convert theoptical signal into an electric signal, then outputs the electricsignal, wherein the circuit on the secondary side includes impedancevariable means in a negative feedback circuit of a current-to-voltageconverting amplifier (an I/V converting amplifier) for amplifying thephotoelectric current produced from the photo-electric conversion by thelight-receiving element, the impedance variable section changing animpedance in accordance with a level of the inputted photoelectriccurrent, wherein the impedance variable section lowers a gain of the I/Vconverting amplifier as the level of the inputted photoelectric currentincreases.

[0017] According to the arrangement, the optical coupling devicerealized as, for example, a photocoupler, which electrically isolatesthe circuit on the primary side from the circuit on the secondary sideby converting the inputted electric signal into the optical signal bythe circuit on the primary side and converting it back into the electricsignal by the circuit on the secondary side, including the impedancevariable section in the negative feedback circuit of thecurrent-to-voltage converting amplifier for amplifying the photoelectriccurrent produced from the photo-electric conversion by thelight-receiving element, the impedance variable section changing animpedance in accordance with a level of the inputted photoelectriccurrent, wherein the impedance variable section raises the gain of theI/V converting amplifier as the level of the inputted photoelectriccurrent decreases and lowers the gain when the level of the inputtedphotoelectric current increases. Because of this, even when the outputsfrom the I/V converting amplifier are subjected to waveform shaping atthe same threshold value, it is possible to minimize pulse widthdistortion caused by the light quantity. This realizes the high-speedtransmission rate of the pulse.

[0018] Moreover, in an arrangement including two sets of the circuits onthe secondary side in which the light-receiving elements are used as aphotodiode and a dummy photodiode for the purpose of eliminating acommon mode noise signal, both gains of the I/V converting amplifiersdecrease with respect to the common mode current due to the noise,thereby restricting the faulty operation, and thus increasing the CMRcharacteristics.

[0019] For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a block diagram showing an electrical arrangement of alight-receiving circuit of an embodiment of the present invention.

[0021]FIG. 2 is a block diagram showing an electrical arrangement of alight-receiving circuit of another embodiment of the present invention.

[0022] FIGS. 3(a) and 3(b) are graphs showing frequency characteristicsof I/V converting amplifiers in the light-receiving circuits of FIGS. 1and 2.

[0023]FIG. 4 is a block diagram showing an electrical arrangement of alight-receiving circuit of still another embodiment of the presentinvention.

[0024]FIG. 5 is a front view showing a configuration of a photodiode anda dummy photodiode in the light-receiving circuit of FIG. 4.

[0025]FIG. 6 is a cross-sectional view showing a structure of a typicalphotocoupler.

[0026]FIG. 7 is a block diagram showing an electrical arrangement of aphotocoupler of a typical art.

[0027]FIG. 8 is a block diagram showing an electrical arrangement of alight-receiving circuit of another conventional art.

[0028]FIG. 9 is a front view showing a configuration of a photodiode anda dummy photodiode in the light-receiving circuit of FIG. 8.

[0029] FIGS. 10(a) and 10(b) are diagrams showing noise applied to thephotocoupler and an output waveform of the photodiode caused by thenoise.

DESCRIPTION OF THE EMBODIMENTS

[0030] An embodiment of the present invention is described as follows,referring to FIGS. 1 through 5.

[0031]FIG. 1 is a block diagram showing an electrical arrangement of alight-receiving circuit 31 of the embodiment of the present invention.The light-receiving circuit 31 is provided with two photodiodes D1 andD2 having identical properties in an identical shape and quantity. Onlythe photodiode D1 is used for receiving the optical signal from thelight emitting element, whereas the other photodiode, that is, thephotodiode D2 is a dummy photodiode by being shielded from light. Thedummy photodiode D2, having its light-receiving face covered with acathode metal wiring 32, is shielded from light where a cathodepotential is applied on the cathode metal wiring 32.

[0032] Output currents from the photodiodes D1 and D2 are converted intovoltage respectively by the I/V converting amplifiers A1 and A2, andcompared from each other by a hysteresis comparator 33, which is adifferential amplifier. In this way, the output from the photodiode D1is subjected to waveform shaping to be a pulse signal. This eliminates acommon mode noise component, thereby realizing the output of high CMRcharacteristics.

[0033] Notably, in the present invention, the I/V converting amplifiersA1 and A2 are amplifiers of nonlinear amplification that carry outnegative feedback respectively via impedance variable circuits Z1 andZ2. The impedance variable circuit Z1 is composed of (a) a seriescircuit of a resistor R1 and a transistor Q1 arranged to have a diodestructure (later described), and (b) a condenser C1, the series circuitand the condenser being provided in parallel between an output and anegative input of the I/V converting amplifier A1. Further, forachieving consistency, also on the dummy photodiode D2 side, theimpedance variable circuit Z2 is composed of (a) a series circuit of aresistor R2 and a transistor Q2 arranged to have a diode structure(later described), and (b) a condenser C2, the series circuit and thecondenser being provided in parallel between an output and a negativeinput of the I/V converting amplifier A2. Note that, here, the abovearrangement satisfies R2=R1, Q2=Q1, and C2=C1.

[0034] Therefore, when a current flowing through the photodiode D1 isIpd, and a bias current of an input terminal of the I/V convertingamplifier A1 is Ib, an impedance Z1 of the impedance variable circuit Z1is expressed as follows:

Z 1=R 1+(kT/q)/(Ib+Ipd),  (1)

[0035] where k is the Boltzmann constant, T is the absolute temperature,and q is an elementary electric charge.

[0036] Accordingly, since an increase of the current Ipd of thephotodiode D1 decreases the impedance Z1, an input of a big signallowers a gain of the I/V converting amplifier A1. Thus, since the I/Vconverting amplifier A1 gives the hysteresis comparator 33 an outputvoltage subjected to logarithmic compression, it is possible to preventan output pulse width from being increased, even when the hysteresiscomparator 33 carries out waveform shaping at a fixed threshold valuefrom the I/V converting amplifier A2. Further, when a common modecurrent due to noise flows through the dummy photodiode D2 and thephotodiode D1, gains of the I/V converting amplifiers A1 and A2 arelowered. This restricts the faulty operation, and thus realizing highCMR characteristics.

[0037] Moreover, when the band of the I/V converting amplifier A1 itselfis wide enough, a cutoff frequency (band) fc of the I/V convertingamplifier A1 is expressed as follows:

fc=1/(2π·C 1·Z 1).  (2)

[0038] Accordingly, when the current Ipd of the photodiode D1 increasesand the impedance Z1 decreases as described above, the cutoff frequencyfc increases. For example, where R1=5 kΩ, C1=0.3 pF, Ib=2 μA, Ipd=1 μA,and T=300K, Z1=13.6 kΩ and fc=39 MHz are given. Here, if the current Ipdincreases to 2 μA, Z1=11.45 kΩ and fc=46.3 MHz are given.

[0039] As described above, according to the equation 2, the band of theI/V converting amplifier Al is widen when according to the equation 1the impedance Z1 is lowered, in proportion to the current Ipd of thephotodiode D1, by inserting the condenser C1 in parallel to the seriescircuit of the transistor Q1 and the resistor R1 that vary the impedanceZ1. This further reduces pulse width distortion caused by quantitychanges of incoming light. Further, this narrows the band of the I/Vconverting amplifier A1 when the current Ipd of the photodiode D1 doesnot flow, thereby helping realization of high CMR characteristics.

[0040] Another embodiment of the present invention is explained asfollows, referring to FIGS. 2, 3(a), and 3(b).

[0041]FIG. 2 is a block diagram showing an electrical arrangement of alight-receiving circuit 41 of the another embodiment of the presentinvention. Since the light-receiving circuit 41 is similar to theabove-described light-receiving circuit 31, the same reference codes areassigned to the corresponding sections, and explanation thereof isomitted here.

[0042] In the light-receiving circuit 31, the transistors Q1 and Q2,which varies impedances in the impedance variable circuits Z1 and Z2,have diode structures in which a collector and a base are connected.Meanwhile, in impedance variable circuits Z1 a and Z2 a in thelight-receiving circuit 41, resistors R1 a and R2 a are respectivelyprovided between the collector and the base, and I/V convertingamplifiers A1 and A2 have frequency characteristics having a peak. Inaddition, a dummy photodiode D2 side is arranged to satisfy R2 a R1 a,as well as R2=R1 that is described above, for achieving consistency.

[0043] Therefore, the equation 1 is expressed as follows:

Z 1=R 1+R 1 a/hFE(Q 1)+(kT/q)/(Ib+Ipd),  (3)

[0044] where hFE(Q1) is a current amplification ratio of the transistorQ1. The current amplification ratio hFE, having frequencycharacteristics, is expressed in complex variables as follows;

hFE(jf)=hFE0/(1+hFE0·(f/fTh)·j),  (4)

[0045] where fTh is called as a transient frequency, which is afrequency to satisfy hFE=1.

[0046] On the other hand, an impedance of a gain resistance, which is avalue that is obtained when Z1 and C1 are connected in parallel, hasfrequency characteristics.

Gain resistance=Z 1//C 1=Z 1/(1+2π·f·C 1·j)  (5)

[0047] Though this calculation is very complicated, when R1=5 kΩ, R1a=10 kΩ, C1=0.3 pF, Ib=2 μA, Ipd=5 μA, T=300K, hFE0=100, and fTh=1 GHz,for example, the calculation of the frequency characteristics of thegain resistance according to the equation 5 gives a result as shown inFIG. 3(a), and thus causing peaking. The peak is eliminated when theresistor R1 a is reduced.

[0048] The frequency characteristics of the I/V converting amplifier A1using the impedance variable circuit Z1 is indicated by the referencecode α1 in FIG. 3(b). Accordingly, an increase of the frequencydecreases hFE (Q1) and increases the impedance Z1 when the impedancevariable circuit Z1 a is used. For this reason, in the frequencycharacteristics, according to the equation 2, peaking of the gain occuraround the cutoff frequency fc, as indicated by the reference code α2.This increases the extension rate of a band with respect to the currentIpd of the photodiode D1, thereby further reducing the pulse widthdistortion due to unevenness of the current Ipd.

[0049] Still another embodiment of the present invention is described asfollows, referring to FIGS. 4, 5, and 9.

[0050]FIG. 4 is a block diagram showing an electrical arrangement of alight-receiving circuit 51 of the still another embodiment of thepresent invention. Since the light-receiving circuit 51 is similar tothe above-mentioned light-receiving circuits 31 and 41, the samereference codes are assigned to the corresponding sections, and thusexplanation thereof is omitted here. Notably, in the light-receivingcircuit 51, an output terminal of the I/V converting amplifier A1 on thephotodiode D1 a side is provided with an offset circuit 52 for adjustingits sensitivity in signal reception. Therefore, because as describedabove, consistency is achieved between the I/V converting amplifiers A1and A2 or the impedance variable circuits Z1 and Z2 (Z1 a and Z2 a),which are respectively provided in the photodiode D1 a side and thedummy photodiode D2 side, a little inconsistency is caused in the outputterminals thereof, here.

[0051] For this reason, the photodiode D1 a and the dummy photodiode D2are formed to have different areas respectively as shown in FIG. 5 so asto cancel the inconsistency, whereas normally they are formed to have anequal area as shown in FIG. 9. The area ratio is about 1:0.9. In thisway, the light-receiving circuit 51 is composed for obtaining the bestCMR characteristics.

[0052] As described above, the optical coupling device of the presentinvention realized as, for example, a photocoupler, is so adapted thatimpedance variable means in a negative feedback circuit of an I/Vconverting amplifier for amplifying the photoelectric current producedfrom the photo-electric conversion by the light-receiving element, theimpedance variable means changing an impedance in accordance with alevel of the inputted photoelectric current, wherein the impedancevariable means raises a gain of the I/V converting amplifier as thelevel of the inputted photoelectric current decreases and lowers thegain when the level of the inputted photoelectric current increases.

[0053] Because of this, even when the outputs from the I/V convertingamplifier are subjected to waveform shaping at the same threshold value,it is possible to minimize pulse width distortion caused by the lightquantity. This realizes the high-speed transmission rate of the pulse.Moreover, in an arrangement including two sets of the circuits on thesecondary side in which the light-receiving elements are used as aphotodiode and a dummy photodiode for the purpose of eliminating acommon mode noise signal, both gains of the I/V converting amplifiersdecrease with respect to the common mode current due to the noise,thereby restricting the faulty operation, and thus increasing the CMRcharacteristics.

[0054] Moreover, an optical coupling device of the present invention isso adapted that a circuit on a primary side includes a light emittingelement for converting an inputted electric signal into an opticalsignal, and a circuit on a secondary side includes (a) a light-receivingelement, which is placed in a vicinity of the light emitting element soas to face the light emitting element, the light-receiving elementreceiving the optical signal and performing photoelectric conversion ofthe optical signal, and (b) an I/V converting amplifier for amplifyingthe photoelectric current subjected to the photo-electric conversion bythe light-receiving element and outputting the photoelectric current,wherein the I/V converting amplifier includes a negative feedbackcircuit having impedance variable means for lowering a gain of the I/Vconverting amplifier as a level of the inputted photoelectric currentincreases.

[0055] According to the arrangement, even when the outputs from the I/Vconverting amplifier are subjected to waveform shaping at the samethreshold value, it is possible to minimize pulse width distortioncaused by the light quantity. This realizes the high-speed transmissionrate of the pulse. Moreover, in an arrangement including two sets of thecircuits on the secondary side in which the light-receiving elements areused as a photodiode and a dummy photodiode for the purpose ofeliminating a common mode noise signal, both gains of the I/V convertingamplifiers decrease with respect to the common mode current due to thenoise, thereby restricting the faulty operation, and thus increasing theCMR characteristics.

[0056] Moreover, it is more preferable that the impedance variable meansis composed of (a) a series circuit of a first resistor and atransistor, and (b) a condenser, the series circuit and the condenserbeing provided in parallel between an output and a negative input of theI/V converting amplifier.

[0057] With this arrangement where the condenser is inserted in parallelto the series circuit of the transistor and the first resistor thatvaries the impedance, the band of the I/V converting amplifier is widenwhen the impedance is lowered, in proportion to the current flowingthrough the light-receiving element. This further reduces pulse widthdistortion caused by quantity changes of incoming light. Further, thisnarrows the band of the I/V converting amplifier when the current of thelight-receiving element does not flow, thereby helping realization ofhigh CMR characteristics.

[0058] Moreover, it is more preferable that the impedance variable meansis composed of (a) a series circuit of a first resistor and atransistor, (b) a condenser, and (c) a second resistor, the seriescircuit and the condenser being provided in parallel between an outputand a negative input of the I/V converting amplifier, and the secondresistor being provided between a collector and a base of thetransistor.

[0059] According to this, when the frequency of the inputtedphotoelectric current increases, the current amplification ratio of thetransistor in the impedance variable means decreases and the impedanceof the impedance variable means increases. For this reason, in thefrequency characteristics of the I/V converting amplifier, peaking ofthe gain occurs around the cutoff frequency. This increases theextension rate of a band with respect to the photoelectric current,thereby further reducing the pulse width distortion caused by unevennessof the photoelectric current.

[0060] Moreover, it is more preferable that the light-receiving elementis composed of a photodiode and a dummy photodiode, respectively havingan I/V converting amplifier so as to eliminate a common mode noisesignal, the optical coupling device including an offset circuit at anoutput terminal of the I/V converting amplifier on the photodiode sidefor adjusting sensitivity in signal reception, wherein the photodiodeand the dummy photodiode are formed to have different areas respectivelyso as to cancel inconsistency in outputs of the I/V convertingamplifiers caused by the offset circuit.

[0061] According to this, the common mode noise signal is eliminated byproviding the dummy photodiode to the photodiode and obtaining adifference between outputs of the I/V converting amplifiers respectivelycorresponding to the photodiode and the dummy photodiode. Moreover, inan arrangement for achieving high CMR characteristics, an area ratiobetween the photodiode and the dummy photodiode are adjusted so that itis possible to cancel the inconsistency in outputs caused by the offsetcircuit provided for adjusting the sensitivity in signal reception.Accordingly, the sensitivity in signal reception can be easily adjusted.

[0062] In order to solve the problems, an optical coupling device of thepresent invention is so adapted that a circuit on a primary sideincludes a light emitting element for converting an inputted electricsignal into an optical signal and a circuit on a secondary side includes(a) a light-receiving element, which is placed in a vicinity of thelight emitting element so as to face the light emitting element, thelight-receiving element receiving the optical signal, and (b) an I/Vconverting amplifier for amplifying the photoelectric current subjectedto the photo-electric conversion by the light-receiving element, whereinthe circuit on the secondary side includes a photodiode and a dummyphotodiode as light-receiving elements, and I/V converting amplifiersrespectively corresponding to the photodiode and the dummy photodiode,and wherein the I/V converting amplifier includes a negative feedbackcircuit having impedance variable means for lowering a gain of the I/Vconverting amplifier as a level of the inputted photoelectric currentincreases.

[0063] According to the arrangement, even when the outputs from the I/Vconverting amplifier are subjected to waveform shaping at the samethreshold value, it is possible to minimize pulse width distortioncaused by the light quantity. This realizes the high-speed transmissionrate of the pulse. Moreover, in an arrangement including two sets of thecircuits on the secondary side in which the light-receiving elements areused as a photodiode and a dummy photodiode for the purpose ofeliminating a common mode noise signal, both gains of the I/V convertingamplifiers decrease with respect to the common mode current due to thenoise, thereby restricting the faulty operation, and thus increasing theCMR characteristics.

[0064] Moreover, it is more preferable that the impedance variable meansis composed of (a) a series circuit of a first resistor and atransistor, and (b) a condenser, the series circuit and the condenserbeing provided in parallel between an output and a negative input of theI/V converting amplifier.

[0065] With this arrangement where the condenser is inserted in parallelto the series circuit of the transistor and the first resistor thatvaries the impedance, the band of the I/V converting amplifier is widenwhen the impedance is lowered, in proportion to the current flowingthrough the light-receiving element. This further reduces pulse widthdistortion caused by quantity changes of incoming light. Further, thisnarrows the band of the I/V converting amplifier when the current of thelight-receiving element does not flow, thereby helping realization ofhigh CMR characteristics.

[0066] Moreover, it is preferable that the impedance variable meansincludes a second resistor provided between a collector and a base ofthe transistor.

[0067] This increases the extension rate of the band with respect to thecurrent of the light-receiving element, thereby further reducing thepulse width distortion caused by unevenness of the current.

[0068] Moreover, it is preferable that an offset circuit is provided atan output terminal of the I/V converting amplifier on the photodiodeside of the I/V converting amplifiers respectively corresponding to thephotodiode and the dummy photodiode, the offset circuit adjustingsensitivity in signal reception, wherein the photodiode and the dummyphotodiode are formed so as to have different areas respectively.

[0069] According to this, the common mode noise signal is eliminated byproviding the dummy photodiode to the photodiode and obtaining adifference between outputs of the I/V converting amplifiers respectivelycorresponding to them. Moreover, in the arrangement for achieving highCMR characteristics, an area ratio between the photodiode and the dummyphotodiode are adjusted so that it is possible to cancel theinconsistency in outputs caused by the offset circuit provided foradjusting the sensitivity in signal reception. Accordingly, thesensitivity in signal reception can be easily adjusted.

[0070] Moreover, in order to solve the problems, a light-receivingcircuit of an optical coupling device of the present invention includesa light-receiving element, which is placed in a vicinity of the lightemitting element so as to face the light emitting element for convertingan inputted electric signal into an optical signal, the light-receivingelement receiving the optical signal and performing photo-electricconversion of the optical signal, an I/V converting amplifier foramplifying the photoelectric current subjected to the photo-electricconversion by the light-receiving element and outputting thephotoelectric current, wherein the I/V converting amplifier includes anegative feedback circuit having impedance variable means for lowering again of the I/V converting amplifier as a level of the inputtedphotoelectric current increases.

[0071] According to the arrangement, even when the outputs from the I/Vconverting amplifier are subjected to waveform shaping at the samethreshold value, it is possible to minimize pulse width distortioncaused by the light quantity. This realizes the high-speed transmissionrate of the pulse. Moreover, in the arrangement including two sets ofthe circuits on the secondary side in which the light-receiving elementsare used as a photodiode and a dummy photodiode for the purpose ofeliminating a common mode noise signal, both gains of the I/V convertingamplifiers decrease with respect to the common mode current due to thenoise, thereby restricting the faulty operation, and thus increasing theCMR characteristics.

[0072] Moreover, in order to solve the problems, a light-receivingcircuit of an optical coupling device of the present invention includesa light-receiving element, which is placed in a vicinity of a lightemitting element so as to face the light emitting element for convertingan inputted electric signal into an optical signal, the light-receivingelement receiving the optical signal, an I/V converting amplifier foramplifying the photoelectric current subjected to the photo-electricconversion by the light-receiving element, wherein the photodiode andthe dummy photodiode compose the light-receiving elements, and the I/Vconverting amplifier respectively corresponds to the photodiode and thedummy photodiode, and wherein the I/V converting amplifier includes anegative feedback circuit having impedance variable means for lowering again of the I/V converting amplifier as a level of the inputtedphotoelectric current increases.

[0073] According to the arrangement, even when the outputs from the I/Vconverting amplifier are subjected to the waveform shaping at the samethreshold value, it is possible to minimize pulse width distortioncaused by the light quantity. This realizes the high-speed transmissionrate of the pulse. Moreover, in the arrangement including two sets ofthe circuits on the secondary side in which the light-receiving elementsare used as a photodiode and a dummy photodiode for the purpose ofeliminating a common mode noise signal, both gains of the I/V convertingamplifiers decrease with respect to the common mode current due to thenoise, thereby restricting the faulty operation, and thus increasing theCMR characteristics.

[0074] Moreover, it is preferable that the impedance variable means iscomposed of (a) a series circuit of a first resistor and a transistor,and (b) a condenser, the series circuit and the condenser being providedin parallel between an output and a negative input of the I/V convertingamplifier.

[0075] With this arrangement where the condenser is inserted in parallelto the series circuit of the transistor and the first resistor thatvaries the impedance, the band of the I/V converting amplifier is widenwhen the impedance is lowered, in proportion to the current flowingthrough the light-receiving element. This further reduces pulse widthdistortion caused by quantity changes of incoming light. Further, thisnarrows the band of the I/V converting amplifier when the current of thelight-receiving element does not flow, thereby helping realization ofhigh CMR characteristics.

[0076] Moreover, it is preferable that the impedance variable meansincludes a second resistor, which is provided between a collector and abase of the transistor.

[0077] This increases the extension rate of the band with respect to thecurrent of the light-receiving element, thereby further reducing thepulse width distortion caused by unevenness of the current.

[0078] Moreover, it is preferable that an offset circuit at an outputterminal of the I/V converting amplifier on the photodiode side of theI/V converting amplifiers respectively corresponding to the photodiodeand the dummy photodiode, the offset circuit adjusting sensitivity insignal reception, wherein the photodiode and the dummy photodiode areformed so as to have different areas respectively.

[0079] According to this, the common mode noise signal is eliminated byproviding the dummy photodiode to the photodiode and obtaining adifference between outputs of the I/V converting amplifiers respectivelycorresponding to them. Moreover, in the arrangement for achieving highCMR characteristics, an area ratio between the photodiode and the dummyphotodiode are adjusted so that it is possible to cancel theinconsistency in outputs caused by the offset circuit provided foradjusting the sensitivity in signal reception. Accordingly, thesensitivity in signal reception can be easily adjusted.

[0080] The invention being thus described, it will be obvious that thesame way may be varied in many ways. Such variations are not to beregarded as a departure from the spirit and scope of the invention, andall such modifications as would be obvious to one skilled in the art areintended to be included within the scope of the following claims.

What is claimed is:
 1. An optical coupling device, in which a circuit ona primary side uses a light emitting element to convert an inputtedelectric signal into an optical signal, whereas a circuit on a secondaryside, in which a light-receiving element is placed in a vicinity of thelight emitting element so as to face the light emitting element, usesthe light-receiving element to accept the optical signal and to convertthe optical signal into an electric signal, then outputs the electricsignal, wherein: the circuit on the secondary side includes impedancevariable means in a negative feedback circuit of a current-to-voltageconverting amplifier (an I/V converting amplifier) for amplifying thephotoelectric current produced from the photo-electric conversion by thelight-receiving element, the impedance variable means changing animpedance in accordance with a level of the inputted photoelectriccurrent, wherein the impedance variable means lowers a gain of the I/Vconverting amplifier as the level of the inputted photoelectric currentincreases.
 2. An optical coupling device, wherein: a circuit on aprimary side includes a light emitting element for converting aninputted electric signal into an optical signal; and a circuit on asecondary side includes (a) a light-receiving element, which is placedin a vicinity of the light emitting element so as to face the lightemitting element, the light-receiving element receiving the opticalsignal and performing photo-electric conversion of the optical signal,and (b) an I/V converting amplifier for amplifying the photoelectriccurrent subjected to the photo-electric conversion by thelight-receiving element and outputting the photoelectric current,wherein the I/V converting amplifier includes a negative feedbackcircuit having impedance variable means for lowering a gain of the I/Vconverting amplifier as a level of the inputted photoelectric currentincreases.
 3. The optical coupling device as set forth in claim 2,wherein: the impedance variable means is composed of (a) a seriescircuit of a first resistor and a transistor, and (b) a condenser, theseries circuit and the condenser being provided in parallel between anoutput and a negative input of the I/V converting amplifier.
 4. Theoptical coupling device as set forth in claim 1, wherein: the impedancevariable means is composed of (a) a series circuit of a first resistorand a transistor, (b) a condenser, and (c) a second resistor, the seriescircuit and the condenser being provided in parallel between an outputand a negative input of the I/V converting amplifier, and the secondresistor being provided between a collector and a base of thetransistor.
 5. The optical coupling device as set forth in claim 1,wherein the light-receiving element is composed of a photodiode and adummy photodiode, respectively having an I/V converting amplifier so asto eliminate a common mode noise signal, the optical coupling devicecomprising: an offset circuit at an output terminal of the I/Vconverting amplifier on the photodiode side for adjusting sensitivity insignal reception, wherein the photodiode and the dummy photodiode areformed to have different areas respectively so as to cancelinconsistency in outputs of the I/V converting amplifiers caused by theoffset circuit.
 6. An optical coupling device, wherein: a circuit on aprimary side includes a light emitting element for converting aninputted electric signal into an optical signal; and a circuit on asecondary side includes (a) a light-receiving element, which is placedin a vicinity of the light emitting element so as to face the lightemitting element, the light-receiving element receiving the opticalsignal, and (b) an I/V converting amplifier for amplifying thephotoelectric current subjected to the photo-electric conversion by thelight-receiving element, wherein the circuit on the secondary sideincludes a photodiode and a dummy photodiode as light-receivingelements, and I/V converting amplifiers respectively corresponding tothe photodiode and the dummy photodiode, and wherein the I/V convertingamplifier includes a negative feedback circuit having impedance variablemeans for lowering a gain of the I/V converting amplifier as a level ofthe inputted photoelectric current increases.
 7. The optical couplingdevice as set forth in claim 6, wherein: the impedance variable means iscomposed of (a) a series circuit of a first resistor and a transistor,and (b) a condenser, the series circuit and the condenser being providedin parallel between an output and a negative input of the I/V convertingamplifier.
 8. The optical coupling device as set forth in claim 7,wherein: the impedance variable means includes a second resistor, whichis provided between a collector and a base of the transistor.
 9. Theoptical coupling device as set forth in claim 6, comprising: an offsetcircuit at an output terminal of the I/V converting amplifier on thephotodiode side of the I/V converting amplifiers respectivelycorresponding to the photodiode and the dummy photodiode, the offsetcircuit adjusting sensitivity in signal reception, wherein thephotodiode and the dummy photodiode are formed so as to have differentareas respectively.
 10. A light-receiving circuit of an optical couplingdevice, comprising: a light-receiving element, which is placed in avicinity of the light emitting element so as to face the light emittingelement for converting an inputted electric signal into an opticalsignal, the light-receiving element receiving the optical signal andperforming photo-electric conversion of the optical signal; an I/Vconverting amplifier for amplifying the photoelectric current subjectedto the photo-electric conversion by the light-receiving element andoutputting the photoelectric current, wherein the I/V convertingamplifier includes a negative feedback circuit having impedance variablemeans for lowering a gain of the I/V converting amplifier as a level ofthe inputted photoelectric current increases.
 11. A light-receivingcircuit of an optical coupling device, comprising: a light-receivingelement, which is placed in a vicinity of a light emitting element so asto face the light emitting element for converting an inputted electricsignal into an optical signal, the light-receiving element receiving theoptical signal; an I/V converting amplifier for amplifying thephotoelectric current subjected to the photo-electric conversion by thelight-receiving element, wherein the photodiode and the dummy photodiodecompose the light-receiving elements, and the I/V converting amplifierrespectively corresponds to the photodiode and the dummy photodiode,wherein the I/V converting amplifier includes a negative feedbackcircuit having impedance variable means for lowering a gain of the I/Vconverting amplifier as a level of the inputted photoelectric currentincreases.
 12. The light-receiving circuit of the optical couplingdevice as set forth in claim 10, wherein: the impedance variable meansis composed of (a) a series circuit of a first resistor and atransistor, and (b) a condenser, the series circuit and the condenserbeing provided in parallel between an output and a negative input of theI/V converting amplifier.
 13. The light-receiving circuit of the opticalcoupling device as set forth in claim 12, wherein: the impedancevariable means includes a second resistor, which is provided between acollector and a base of the transistor.
 14. The light-receiving circuitof the optical coupling device as set forth in claim 11, comprising: anoffset circuit at an output terminal of the I/V converting amplifier onthe photodiode side of the I/V converting amplifiers respectivelycorresponding to the photodiode and the dummy photodiode, the offsetcircuit adjusting sensitivity in signal reception, wherein thephotodiode and the dummy photodiode are formed so as to have differentareas respectively.